Forestry 485 lecture 3 1

Forestry 485Lecture 3-1: Urea and Phenol Formaldehyde Adhesive Resins... Thermosetting Resins for Wood Composites Panel Products - three primary resins in use in North America: Urea for

Forestry 485

Lecture 3-1: Urea and Phenol Formaldehyde Adhesive Resins

Thermosetting Resins for Wood

Composites

Panel Products - three primary resins in use

in North America:

Urea formaldehyde (water resistant bond), used exclusively for interior applications

Phenol formaldehyde (fully exterior, waterproof

bond), used almost exclusively for exterior

application and/or structural products

Polymeric MDI (water resistant to water proof), used mainly as a core resin in OSB

Urea Formaldehyde (UF)

Synthesis

Cure chemistry

UF Synthesis

UF is an “amino resin” or

“amine-formaldehyde” class of adhesive

UF is synthesized by the reaction of urea and formaldehyde to obtain stable, but

reactive intermediates

Storage stability due to reduced reactivity

of intermediates in alkaline conditions

(>pH 7)

UF Synthesis

Mild alkali (pH>7)

pH is used to control reaction rate Occurs very rapidly under acidic conditions

Production of the reactive intermediates is monitored and controlled by viscosity

Note methylolation of urea residue

UF Synthesis, continued

Other intermediates, such are dimethylol urea, are also formed

Again, viscosity is monitored to indicate the

stage of the synthesis Increasing viscosity is correlated to polymer growth; this indicates

how “advanced” the resin is

pH is maintained above 7 to slow

polymerization and thereby increase storage life

UF Cure Chemistry

Polymerization of the reaction intermediates

chain length and molecular weight

“condensation reaction”

and ultimately to solid

mechanical strength of adhesive bond

UF Condensation Reaction: Favored by acidic

conditions

Result: Cured

network

(cross-linked) molecular

structure

Condensation

may be linear

UF Condensation

Controlled by:

 Temperature (150 o C cure temperature)

 Time

 pH (>7, stable; <7, condensation)

Reaction accelerated by “curing agent” or “hardener”

Reading 3-1c

 Ratio of formaldehyde to urea

Molar ratio varies from just >1.0 to approx 1.5 Lower molar ratios favored in contemporary formulations to reduce formaldehyde emissions Most are <1.1

Formaldehyde scavengers may be used (see Reading 3-1d)

Phenol formaldehyde (PF)

adhesive resins

Two major types:

 Resole (cross-linking thermosetting resin)

 Novolac (resin with some thermoplastic properties)

Synthesis

Condensation reactions (resin cure)

PF Synthesis: Resoles

Two stages:

 Methylolation of phenol to produce methylol phenols First stage, or “A” stage, is alkaline catalyzed for the synthesis of resoles

 Alkaline A stage, P/F ratio=1:1 to 1:3 (i.e., molar

EXCESS of formaldehyde)

 B stage is the condensation of intermediates to

produce water insoluble, but fusible products

Reaction is monitored by viscosity and terminated

prior to complete polymerization.

Resole “A”

Methylolation may occur at ortho, meta, or para position.

Resole “B”

Note that an excess of formaldehyde is used This promotes full methylolation

of the phenol moieties.

Resole “C” (curing)

Process cure temperature typically

heat is needed to cure.

Three-dimensional cross-linked structure is formed

Water

Novolac Synthesis

P/F ratio is > 1.0 (note error in figure on p 78 of Marra; in fact, note somewhat unorthodox

means of representing P:F ratio on p 77 & 78) Acid A stage, P/F ratio=1:1 to 1:0.6

Few methylols are formed; results in linear,

rather than cross-linked structure

Has indefinite shelf life

Soluble, (partially) thermoplastic

Hardener in the form of additional formaldehyde

is needed for cure

Novolac synthesis

Formaldehyde donors for cure:

Formalin Paraformaldehyde

Resole vs Novolac

Resole

weight “impregnating” resin

(use for making Impreg,

Compreg, and laminating

materials)

(adhesive) resin

polymerize

storage (limited shelf life)

Novolac

thus much less cross-linking (linear structure)

supplied to cure, along with heat

thermoplastic

flake form (dry)